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QUESTION: 1

The degree of dissociation of water in a 0.1 M aqueous solution of HC1 at a cedain temperature t°C is 3.6 x 10^{-15}. The temperature t must be :

Solution:

QUESTION: 2

Which one is the correct expression below for the solution containing **'n' number of weak acids?**

Solution:

QUESTION: 3

The pH of glycine at the first half equivalence point is 2.34 and that at second half equivalence paint is 9.60. At the equivalence point (The first inflection point) The pH is :

Solution:

QUESTION: 4

The pK, of a weak acid if titration progress is monitored as follows

Solution:

In the horizontal part of the curve, the solution is acting as buffer. At the inflection point, [salt]=[acid]. Hence, pH = pK_{a }= 3.88.

QUESTION: 5

A 1.458 g of Mg reacts with 80.0 ml of a HCI solution whose pH is -0.477. The change in pH after all Mg has reacted. (Assume constant volume. Mg = 24.3 g/mol.)(log 3 = 0.477)

Solution:

Mg (aq) + 2HCl(aq) → MgCl_{2 }(aq) + H_{2 }

1.458/24.3

Millimoles of HCl = 3 x 80 = 240 mM

Moles of HCL after reaction = 240 - 60 x

New Molarity = 120/80 = 1.5 M

pH = -log[H^{+}] = -log 1.5 = -0.176

change is pH = -0.176 - (-0.477) = 0.3

QUESTION: 6

Equal volume of two solution having pH = 2 and pH = 10 are mixed together at 90°C. Then pH of resulting solution is : (Take Kw at 90°C = 10^{-12})

Solution:

kW = 10^{-12}

Therefore range of pH scale. 0 to -log10^{-12}

= 0 to 12

Since we have a strong acid and strong base mixture, the resultant solution will be neutral. So, the solution will have pH = 6

QUESTION: 7

Find the ApH(initial pH -final pH) when 100 ml 0.01 M HCI is added in a solution containig 0.1 m moles of NaHCO_{3} solution of negligible volume ( Ka_{i} =10^{-7}, Ka, =10^{-11} for H_{2}CO_{3}) :

Solution:

pH of NaHCO_{3} solution = 9

Now H+ + HCO_{3} → H_{2 }CO_{3}

∴ no. of m mole of HCl remaining = 1 - 0.1= 0.9 m mole

∴ pH = -log (9 x 10^{-3}) = -2 log 3 + 3

QUESTION: 8

The ionization constant of benzoic acid is 6.46 x 10^{-5} and lc for silver benzoate is 2.5 x 10^{-13}. How many times silver benzoate more soluble in a buffer of pH = 3.19 as compared to its solubility in pure water?

Solution:

The correct answer is Option A.

K_{a} of PhCOOAg= 6.45×10^{-5}

Ksp of PhCOOAg= 2.5×10^{-13}

pH of buffer = 3.19

∴[H^{⊕}] =Antilog(−3.19)

=Antilog(−3−0.19+1−1)

=Antilog(4.81)

=6.45×10^{-4} M

Let S_{buffer}(solubility of salt in a buffer)

=[1+10]^{1/2}

=(11)^{1/2}

=3.317

Thus, silver benzoate is

3.317 times more soluble at lower

pH than in water.

QUESTION: 9

30 ml of 0.06 M solution of the protonated form of an anion acid methonine (H_{2}A+) is treated with 0.09 M NaOH. Calculate pH after addition of 20 ml of base. pKa, = 2.28 and pKa_{2} = 9.2.

Solution:

QUESTION: 10

A certain acid-base indicator is red in acid solution and blue in basic solution 75% of the indicator is present in the solution in its blue form at pH = 5. Calculate the pH at which the indicator shows 90% red form?

Solution:

QUESTION: 11

Ionisation constant of HA (weak acid) and BOH (weak base) are 3.0 x 10^{-7} each at 298K. The percent degree of hydrolysis of BA at the dilution of 10L is :-

Solution:

B+ + A- + H2O ⇋ BOH + HA

1-α 1-α α α

α/1-α = √kHydrolysis = √kWater/kacid

kbase

= √(10-14)/(3×10-7)2

= ⅓

Or α = 0.25

Therefore, %age dissociation = 25%

QUESTION: 12

Which of the following concentrations of NH_{4}^{+} will be sufficient to prevent the precipitation of Mg(OH)_{2} from a solution which is 0.01 M MgCl_{2} and 0.1 M NH_{3} (aq).

Given that : K_{5} of Mg (OH)_{2} = 2.5 x 10^{-11} and K_{b} for NH_{3}(aq) = 2 x 10^{-6}.

Solution:

The expression for the solubility product of magnesium hydroxide is K_{sp}

[Mg^{2+}][OH^{-}]^{2}

Thus 2.5×10-11 = 0.01 × [OH-]2

[OH-] = 5×10^{-5}

The expression for dissociation constant of base is K_{b} = [NH_{4}^{+}][OH^{-}]/[NH_{3}]

Thus = 2×10^{-5} = [NH_{4}^{+}](5×10^{-5})/0.1

Hence, [NH_{4}^{+}] = 0.04 M

QUESTION: 13

Calculate the molar solubility of zinc tetrathiocyanato-N-mercurate (II) if its K_{sp} = 2.2 x 10^{-7}.

Solution:

QUESTION: 14

An acid-base indicator which is a weak acid has a pK_{a} value = 5.45. At what cocentration ratio of sodium acetate to acetic acid would the indicator show a colour half-way between those of its acid and conjugate base forms? pK_{a} of acetic acid = 4.75. [log 2 = 0.3]

Solution:

QUESTION: 15

The indicator constant of phenolphthalein is approximately 10^{-10}. A solution is prepared by adding 100.01 c.c. of 0.01 N sodium hydroxide to 100.00 c.c. of 0.01N hydrochloric acid. If a few drops of phenolphthalein are now added, what fraction of the indicator is converted to its coloured form?

Solution:

QUESTION: 16

A certain mixture of HCI and CH_{3} - COOH is 0.1 M in each of the acids. 20 ml of this solution is titrated against 0.1M NaOH. By how many units does the pH change from the start to the stage when the HCI is almpst completely neutralised? K_{a} for acetic acid = 1.8 x 10^{-6}.

Solution:

QUESTION: 17

A buffer solution is made by mixing a weak acid HA (K_{a} = 10^{-6}) with its salt NaA in equal amounts. What should be the amount of acid or salt that should be added to make 90 ml of buffer solution of buffer capacity. 0.1 ?

Solution:

For buffer capacity of 0.1 we should have

where is concentration of weak acid or salt in the buffer solution

So, c+0.1 = 10c-1,

so 9c = 1.1 or c= 1.1/9

So, moles required for 90 solution = moles = 11 m moles

QUESTION: 18

A sample of water has a hardness expressed as 80 ppm of Ca^{2+} . This sample is passed through an ion exchange column and the Ca^{2+} is replaced by H^{+}. What is the pH of the water after it has been so treated? [Atomic mass of Ca = 40]

Solution:

10^{6} ml water contains 80gm of ca^{2+} = 80/40 moles = 2 moles of ca^{2+} = 2 x 2 moles of H^{+} ions so

10^{3} ml of H_{2}O will have = 4 x 10^{-3} moles of h+ ions

so pH = 3 - log 4 = 3-0.6 = 2.4

QUESTION: 19

**Statement-1 : **solubility of BaSO_{4} in 0.1 M Na_{2}SO_{4} is 10^{-9} M hence its K_{8} is 10^{-18}.

**Statement-2 : **because for BaSO_{4} K_{s}p = (s)2 [symbols have their usual meanings].

Solution:

QUESTION: 20

**Statement-1 : **It is difficult to distinguish the strengths of the strong acids such as HCI, H_{2}SO_{4}, HNO_{3},HBr, HI or HCIO_{4} in dilute aqueous solutions.

**Statement-2 : **In dilute aqueous solution all strong acids donate a proton to water and are essentially. 100% ionised to produce a solution containing H_{3}0+ ions plus the anions of strong acid .

Solution:

All are strong (α = 1) (leavelling effect).

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